Vite build fails when base option has %

[albatroez]

Describe the bug

When I put a %%%TEMPLATE%%% in the base option in vite.config.js the build starts to fail. This was working before vite v5.4.0.

I'd expect to get a warning but the build still passes. Use case is I'd like to serve multiple versions of the same app with different base urls. The %%%TEMPLATE%%% is later replaced across files with my own script to create those multiple versions. I suppose this could be done with relative path, but should the build fail like that?

Reproduction

https://stackblitz.com/edit/vitejs-vite-jsfwq5kp?file=package.json

Steps to reproduce

Run npm install followed by npm run build

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 18.20.3 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.2.3 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    vite: 5.4.0 => 5.4.0

Used Package Manager

npm

Logs

Click to expand!

(!) "base" option should start with a slash.
error during build:
URIError: URI malformed
    at decodeURI (<anonymous>)
    at resolveConfig (file:///home/projects/vitejs-vite-jsfwq5kp/node_modules/vite/dist/node/chunks/dep-NjL7WTE1.js:66263:18)
    at async build (file:///home/projects/vitejs-vite-jsfwq5kp/node_modules/vite/dist/node/chunks/dep-NjL7WTE1.js:65160:18)
    at async CAC.eval (file:///home/projects/vitejs-vite-jsfwq5kp/node_modules/vite/dist/node/cli.js:843:5)e

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